The present invention relates to an amplifier using a transistor for amplification and, more particularly, to an amplifier for amplifying an AC signal by class "C" amplification.
It has been customary with an amplifier of the type described to include a transistor for amplifying an AC signal by class "C" amplification, and an input and an output matching circuit for matching respectively the input to and the output from the transistor with respect to impedance. Under the same circuit conditions, the amplification and output power of the conventional amplifier depend mainly on the junction or chip temperature .theta. of the transistor. Specifically, the output power increases when the junction temperature .theta. is low or decreases when it is high. Therefore, when an AC signal is applied to the amplifier, the amplifier initially produces a great output since the junction temperature .theta. of the transistor is substantially equal to the ambient temperature. However, as the junction temperature .theta. rises with the elapse of time due to heat generated in the transistor, the output of the amplifier decreases continuously until the temperature elevation (sometimes higher than 100.degree. C.) reaches thermal equilibrium. Let this phenomenon be referred to as an overshoot. The size and duration of the overshoot depend on the rated power of the transistor, the capacity of a radiator, the output power, and so forth. The duration of the overshoot is determined by a thermal time constant and may be as long as several tens of milliseconds to several minutes. The thermal time constant is determined by various parameters including the above-mentioned parameters.
The overshoot broadens the frequency spectrum of an AC signal. Therefore, when the amplifier is implemented as the final stage amplifier of a radio transmitter, the overshoot adversely affects the communication facilities of the other systems. To eliminate this problem, a filter for removing spurious signals have to be added to the output side of the amplifier. Moreover, assume that the output of the amplifier is a signal to be transmitted to a repeater amplifier whose range of input signal levels has little margin, as in satellite communication. Then, the overshoot is apt to saturate the repeater amplifier, thereby obstructing expected communication.